Help-Can I Mix Sugarshacks Bc With...a Or B?

I need to make a batch of icing tonight, (last minute) and do not have enough time to get to the store to buy more Hi-Ratio shortening.I have a 6 qt KA.I have 2 lbs (4 cups) of Hi Ratio.6qt recipe calls for 3 lbs of hi ratio...

Which will give me the best outcome for a crusting BC ...for the 3rd pound...do you think..A) one pound butter orB) one pound of regular wal mart brand shortening (with trans fats)

Not sure what the end results for either option would be and I only have ingredients for one shot.In general, does anyone know the difference between why I would or would not use one over the other (Butter vs Shortening)???